The invention relates to magnetic thin film disks and methods for their fabrication and more particularly to magnetic thin film media having nonuniform magnetic properties due to nonuniform composition and methods for creating the nonuniform composition.
A typical prior art head and disk system 10 is illustrated in FIG. 1. In operation the magnetic transducer 20 is supported by the suspension 13 as it flies above the disk 16. The magnetic transducer 20, usually called a xe2x80x9cheadxe2x80x9d or xe2x80x9cslider,xe2x80x9d is composed of elements that perform the task of writing magnetic transitions (the write head 23) and reading the magnetic transitions (the read head 12). The electrical signals to and from the read and write heads 12, 23 travel along conductive paths (leads) 14 which are attached to or embedded in the suspension 13. Typically there are two electrical contact pads (not shown) each for the read and write heads 12, 23. Wires or leads 14A, 14B, 15A, 15B are connected to these pads and routed in the arm 13 to the arm electronics (not shown). The magnetic transducer 20 is positioned over points at varying radial distances from the center of the disk 16 to read and write circular tracks (not shown). The disk 16 is attached to a spindle 18 that is driven by a spindle motor 24 to rotate the disk 16. The disk 16 comprises a substrate 26 on which a plurality of thin films 21 are deposited. The thin films 21 include ferromagnetic materials in which the write head 23 records the magnetic transitions in which information is encoded.
FIG. 2 illustrates a section of a prior art disk 16. The conventional substrate 26 is a conductive disk of AIMg with an electroless coating of NiP which has been highly polished. The thin films 21 on the disk 16 conventionally include a chromium or chromium alloy underlayer (s) 31 which is deposited on the substrate 26. The recording layer (s) 33 is (are) based on various alloys of cobalt, nickel and iron. For example, a commonly used alloy is CoPtCr. Additional elements such as tantalum and boron are often used to magnetically isolate the grains. A protective overcoat layer 35 is used to improve wearability and corrosion. While instructive, the three film disk described above does not exhaust the possibilities. Various seed layers (not shown), multiple underlayers (not shown) and laminated magnetic films (not shown) have all been described in the prior art. In addition, other materials besides AIMg are utilized as substrates.
When a magnetic disk 16 is designed for a future disk drive 10, a target coercivity range is determined based on the overall system requirements. For example, an upper limit on the coercivity is set by the write head""s 23 ability to induce transitions in the magnetic film 33. Therefore, part of the disk designer""s task is to obtain a specific coercivity range rather than the highest possible coercivity. One method of adjusting the coercivity follows from the fact that the platinum content of the magnetic film 33 is known to directly affect the film coercivity. Within limits, a marginal change in the platinum content will directly affect the coercivity by a predictable amount. The composition of the magnetic film 33 mirrors the composition of the sputtering target to a good level of accuracy, so a marginal increase in the platinum content in sputtering target is reflected in the deposited film.
At a macro level it is desirable for the coercivity of the magnetic film 33 to be fairly uniform both radially and circumferentially. However, there are factors at work in a disk drive which may make it desirable to have subtle radial gradients in the coercivity. For example, at constant rotation speed, the flying height of the transducer 20 above the disk 16 may vary from the inner diameter (ID) to the outer diameter (OD) of the disk 16. The flying height directly affects the field strength generated by the write head 23 in the magnetic film 33. The linear velocity (for constant rpm) is higher at the OD than the ID. This implies that the head flies higher at the OD as compared to the ID. This presents a writability problem at the OD.
Films are grown by sputtering from alloy targets whose compositions are optimized to provide the desired magnetic properties. The target material is held at a negative voltage to provide acceleration for the positively charged sputter gas ions (typically Ar). The ground potential for this arrangement is normally the chamber walls. The substrate is not grounded. Current-art sputtering systems used for fabricating magnetic disks 16 provide also the capability of providing negative or positive bias to the disk substrate. The voltage used is typically on the order of xe2x88x92300 volts.
A method of influencing variations in composition of thin films is described. The elemental plasma field distribution in sputtering systems is manipulated by generating a nonuniform electric field along a surface of the substrate to alter the composition by differentially re-sputtering the target elements. The nonuniform electric field is applied by one or more electrodes in contact with a conductive surface or by using an RF bias signal. The nonuniform electric field is used to modulate the kinetic energy of the ions generated in the plasma which strike the thin film""s surface. Since the kinetic energy and the mass of the sputtering gas ions and neutrals affect the re-sputtering rate, the nonuniform electric field differentially affects the elements being deposited according to mass. By applying varying electric potentials at a plurality of points on a conductive surface of a substrate, the electric field across the surface of the substrate can be modulated in a variety of patterns. For example, the field can be varied along the circumferential and/or radial direction of a disk. In the preferred embodiment a radial voltage gradient is applied to a conductive surface of a disk on which a magnetic thin film is being formed to radially modulate the platinum content of the magnetic film. Modulating the radial platinum content in turn modulates the radial coercivity.